It is well known that there is an increasing shortage of natural gas (chiefly methane) in the United States and there is a generally limited supply of natural gas throughout the world. For this reason, attention is being directed to a substitute or supplement for natural gas. Natural gas suitable for distribution to residential, commercial and industrial consumers is characterized by heating values ranging from about 900 to about 1100 B.t.u./s.c.f. and by a high methane content, normally 80 percent by volume or greater. Such natural gas often contains a small amount of ethane and propane. Therefore, in order to provide a suitable substitute or supplement for natural gas, such substitute or supplement gas should consist essentially of methane or a gaseous hydrocarbon mixture having a high methane content with only small amounts of ethane and propane.
The synthesis of hydrocarbons by hydrogenating carbon monoxide is not a new concept. In fact, the synthesis of methane by hydrogenating carbon monoxide was first described by P. Sabatier and J. B. Senderens in 1902 (Compt. Rend. 134, 514 and 689 [1902]). Higher boiling hydrocarbons were obtained from carbon monoxide and hydrogen in the early 1920's by F. Fischer and H. Tropsch (Chem. Ber. 56, 2428 [1923]). While, at the present time, processes are available for producing a full range of hydrocarbons by hydrogenating carbon monoxide, the economics of such processes has mitigated against their wide-spread commercialization. The products obtained in the catalytic hydrogenation of carbon monoxide can be one or more materials selected from hydrocarbons, alcohols, aldehydes, ketones, esters and fatty acids of almost any chain length, degree of saturation and structure. The relative extent to which one or more of these products is obtained can be controlled to some extent by the selection of the catalyst composition and operating conditions. Catalysts which heretofore have been of special interest in the synthesis of organic compounds from carbon monoxide and hydrogen are those wherein the metal component is selected from iron, cobalt, nickel, ruthenium, zinc and thorium. The behavior of these catalysts in hydrogenating carbon monoxide is dependent to a large extent upon the presence of chemical and structural promotors, upon the method used in preparing the catalyst, upon the catalyst surface conditions, upon the reaction conditions and upon the nature or make-up of the feed gas mixture, i.e., synthesis gas charged to the reaction system.
Nickel has been used as a catalyst for the synthesis of methane according to the reaction EQU CO + 3H.sub.2 .revreaction.CH.sub.4 + H.sub.2 O (1)
which proceeds from left to right at temperatures below about 500.degree. C. and in the opposite direction at higher temperatures.
Cobalt admixed with thorium dioxide and magnesium oxide, as promotors, and kieselguhr, as a carrier, has been used as a catalyst for the synthesis of higher aliphatic hydrocarbons. (F. Fischer and H. Tropsch, Brennstoff-Chem. 7, 97 [1926]; and F. Fischer and H. Pichler, Brennstoff-Chem. 20, 41, 221 and 247 [1939]).
Iron has been used as a catalyst for the synthesis of aliphatic and aromatic hydrocarbons. In the past, alkali has been used as a promotor when the catalyst contains iron. The alkali is reported to influence surface conditions of the catalyst and to enhance the production of higher molecular weight products. In the early work conducted by F. Fischer and H. Tropsch, alkali-promoted iron-copper catalysts were employed in producing high boiling (gasoline range) hydrocarbons. (F. Fischer and H. Tropsch, Brennstoff-Chem. 9, 21 [1928]). The promoting effect of alkali to iron catalysts was believed to be the result of the formation in its presence of ferric oxide (Fe.sub.2 O.sub.3) and the prevention of its transition to the less active magnetic iron oxide (Fe.sub.3 O.sub.4). (G. LeClerc, Compt. Rend 207, 1099 [1939]).
In accordance with the present invention, the presence of alkali in the catalyst is kept at a minimum since it is believed that the presence of alkali in the catalyst of the invention causes the catalyst to fuse and thus materially decrease the surface available for catalytic purposes.
Sintered iron catalysts have previously been used in preparing branched-chain paraffins. These catalysts have been prepared by reducing precipitated iron-alumina catalysts at 1550.degree. F. (British Pat. No. 473,932 [1937]; British Pat. No. 474,448 [1937]; and British Pat. No. 496,880 [1938]).
Ruthenium and ruthenium-containing catalysts have been used in the synthesis of high-melting waxes from hydrogen and carbon monoxide (H. Pichler, Brennstoff-Chem. 19, 226 [1936]; H. Pichler and H. Buffleb, Brennstoff-Chem. 21, 257, 273 and 285 [1940]). Other Group VIII metals, i.e., rhodium, palladium, osmium, iridium and platinum have been less satisfactory than ruthenium (U.S. Pat. No. 1,628,190 [1927]). The effect of pressure upon the yield and type of products with ruthenium catalysts is very pronounced.
Zinc oxide and mixtures of zinc oxide with chromic oxide have been used as catalysts for synthesizing methanol from hydrogen and carbon monoxide at temperatures above 300.degree. C. and pressures above 200 atmospheres. (H. Pichler, Brennstoff-Chem. 33, 289 [1952]).
Oxide catalysts, in general, show a smaller degree of activity toward carbon monoxide plus hydrogen than the metal catalysts. On the other hand, metal catalysts, e.g., nickel, cobalt, iron and ruthenium, are more sensitive to sulfur and sulfur compounds than oxide catalysts.
Prior processes for hydrogenating carbon monoxide to methane and other low boiling hydrocarbons have required hydrogen to carbon monoxide ratios in the order of about 3:1 (see equation 1 hereinabove). Therefore, in many instances, it has been necessary to increase the hydrogen content of synthesis gas by the so-called water gas shift reaction, i.e., EQU CO + H.sub.2 O .fwdarw.CO.sub.2 + H.sub.2 ( 2)
the carbon dioxide formed in the water gas shift reaction is then removed by compressing the gas and scrubbing it with water or by reacting it with ethanolamines. The hydrogen thus obtained is used according to prior processes to increase the hydrogen to carbon monoxide ratio in synthesis gas to an amount of about 2:1 to 3:1, preferably the latter, i.e., about 3:1.
Coal has been used in the production of synthetic or substitute natural gas (SNG) comprising low boiling hydrocarbons according to the Lurgi process as described by Paul F. H. Rudolph in Chemical Age of India, 25, 289-299 (1974). In the Lurgi process for producing SNG from coal, five separate steps are required: (1) pressure gasification of coal to recover gaseous products and remove ash and tar; (2) crude gas shift conversion wherein steam is reacted with some carbon monoxide to form carbon dioxide and hydrogen, the latter being used to increase the hydrogen to carbon monoxide ratio in the synthesis gas; (3) Rectisol gas purification wherein organic solvents remove impurities from the gas; (4) methane synthesis where the carbon monoxide and hydrogen are reacted to produce methane; and (5) a Phenolsolvan process for treating the gas liquor from coal gasification to remove water-soluble components, e.g., phenols, ammonia and fatty acids.
In accordance with the present invention, a sulfur resistant catalyst is provided for the hydrogenation of carbon monoxide to a gaseous hydrocarbon mixture having a high methane content (at least 80 percent by volume methane on a carbon dioxide-free basis) wherein the H.sub.2 :CO ratio can be 1:1. Since this ratio is frequently obtained when coal is subjected to complete gasification, there is no need in the process of the present invention to employ a water gas shift reaction such as that used in the Lurgi process. While coal is an economic source of synthesis gas for use as feed gas in the process of the present invention, the synthesis gas can be obtained from any carbonaceous material which can be decomposed to hydrogen and carbon monoxide. Examples of such materials are fossil fuels such as natural gas, bituminous coal, lignite, oil shale, crude oil and residual fuel oils. For the most part, synthesis gas has been obtained from natural gas or coal. The theoretical ideal synthesis gas reaction may be represented as follows: EQU C + H.sub.2 O .fwdarw.CO + H.sub.2 ( 3)
one of the impurities frequently present in synthesis gas obtained in the gasification of coal is sulfur or compounds of sulfur. As indicated hereinabove, metal catalysts such as nickel, cobalt, iron and ruthenium are poisoned by sulfur and sulfur compounds. Thus, synthesis gas containing sulfur or sulfur compounds has previously been subjected to a desulfurization process prior to being converted into hydrocarbons. One such process is the Girbitol process as described by C. B. Ames, Mines Magazine 32, 508 (1942). Other desulfurization processes include (1) the iron oxide process (C. C. Hall and A. R. Powell, Office of Technical Services Report No. PB288, Department of Commerce, Washington, D.C.); (2) the "Alkazid Process" (Lorenzen Gerhard and Leithe, Gas and Wasserfach 86, 313 [1943]) in which an alkaline organic compound absorbs hydrogen sulfide and then is steam-stripped for reuse (H. A. Schade, E. Foran and R. C. Aldrich, Office of Technical Services Report No. PB373, Department of Commerce, Washington, D.C.); and (3) F. Fischer and H. Tropsch desulfurization by catalytic reduction of sulfur compounds to hydrogen sulfide (British Pat. No. 254,288 [1925]; British Pat. No. 282,634 [1926]; Canadian Pat. No. 266,382 [1926]; and German Pat. No. 558,558 [1926]).
A process for decomposing organic sulfur compounds to hydrogen sulfide by passing the gas at a temperature above 300.degree. C. over a mixture of alkali metal carbonates and iron oxide is disclosed by Studien and Verwertungs G. m. b. H. in British Pat. No. 469,933 (1937) and German Pat. No. 651,462 (1937). In still another process I. G. Farbenindustrie A.-G. has disclosed a process for decomposing organic sulfur compounds to hydrogen sulfide simultaneously with the water gas shift reaction (U.S. Pat. No. 1,695,130 [1928].